Ball bearings and roller bearings have been used as bearings in motors mounted in hard disk drives (HDDs) and the like. However, due to demand for smaller motors, lower vibration/lower noise, etc., fluid dynamic bearings and oil-impregnated sintered bearings, both types of slide bearings, have recently been developed and put into practical use.
A fluid dynamic bearing supports a rotating shaft by the oil film pressure of a lubricating oil present in the gap between the outer circumferential surface of the shaft and the inner circumferential surface of a sleeve. Dynamic pressure grooves are provided in at least one of the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve, and the sliding surface of the rotating shaft is supported in a floating manner by a lubricating oil film formed by the dynamic pressure effect.
Additionally, there is an oil-impregnated sintered bearing in which a porous body made of sintered metal etc. is impregnated with a lubricating oil or a lubricating grease to impart a self-lubricating function, thereby supporting a rotating shaft; there is also a dynamic-pressure-type oil-impregnated sintered bearing, which is an oil-impregnated sintered bearing having dynamic pressure grooves provided in the bearing surface thereof.
Along with the enhanced performance of audiovisual and office automation equipment, increased use of cellular phones, and the like, fluid bearing motors have been put into use. Due to recent strong demand for the speeding up and downsizing of motors, fluid bearings have been required to achieve lower torque. To meet the demand for lower torque, lubricating base oils with relatively low viscosity have been selected. As lubricating base oils with low viscosity, lubricating base oils for fluid bearings, containing a synthetic hydrocarbon-based lubricating base oil such as a poly-α-olefin or containing an ester-based lubricating base oil such as an aliphatic dibasic acid diester, neopentyl-type polyol ester, or fatty acid monoester, have been proposed (Patent Literature 1 to 8).
Among these, many ester-based lubricating base oils, which have excellent viscosity characteristics, heat resistance, low-temperature fluidity, etc., are used as lubricating base oils for fluid bearings.
There are several types of ester-based lubricating base oils having different viscosity characteristics, heat resistance (evaporation resistance), low-temperature fluidity, etc. In addition, the heat resistance (evaporation resistance) of ester-based lubricating base oils tends to become poorer as the viscosity becomes lower. Thus, the mere selection of, in order to reduce the torque of fluid bearings, an ester-based lubricating base oil with a viscosity lower than that of a conventional lubricating base oil leads to impaired heat resistance (evaporation resistance), resulting in reduction in the durability of the fluid bearings.
Additionally, when used as lubricating base oils for fluid bearings, existing ester-based lubricating base oils gradually decompose during operation of fluid bearing motors; therefore, ester-based lubricating base oils are problematic when fluid bearing motors are used for a long period of time.